Ground water with total dissolved solids (TDS) ranging from 10 to >50 g/L exists at depths between 90 and 140 m in Devonian oil-bearing formations in the shallow Michigan Basin of southwestern Ontario. These formations comprise mainly limestone and dolomite, and the sources of the saline ground water have not been identified previously. Isotopic and major-ion data from ground water from Devonian oil-bearing formations in southwestern Ontario indicate that the highest-salinity fluids were emplaced locally from depths of several hundred meters. During the later stages of, or following, the Wisconsinan glaciation, saline water and petroleum were also emplaced into overlying Pleistocene clay-rich glacial deposits. Isostatic rebound leading to fracturing and enhanced formation permeability due to matrix expansion was probably the primary mechanism that enabled these saline fluids to migrate into discrete areas of the Devonian and Pleistocene formations. Variations in ground-water salinity from <10 to <50 g/L TDS over distances of a few hundred meters indicate that this cross-formational fluid flow from depth probably occurred along discrete fractures. Stable-isotope data coincident with the local meteoric water line indicate that leakage of moderately saline, recently recharged meteoric water has occurred since petroleum production began in the last century.

The geochemical data presented here support a model involving cross-formational fluid flow from depth occurring vertically on the scale of several hundred meters since glaciation. The hydrogeologic regime in this shallow basinal system must, therefore, be viewed as dynamic, rather than static, to depths of at least 500 m over a time frame of <10 000 yr.

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